(Invited) Inhomogeneity of Barrier Layer Inducing Irregularity of Porous Anodic Oxide Film on Aluminum

Abstract

Porous anodic oxide films formed on various metals, especially anodic alumina, have attracted huge attention as a starting material in the fabrication of nano-devices because of the high controllability of porous structures in nanometer scale such as pore size, pore interval, as well as pore depth by changing electrolysis condition in addition to their high regularity of pore arrangement. In recent years, mechanism of self-ordering of anodic alumina films has been intensely studied. We proposed anodizing under high electric field strength as a key factor for self-ordering of pore arrangement (1). However, only a few reports are found on the mechanism of irregularity or defects formation of porous anodic oxide films. For example, an regular deposition of metals into all anodic pores are often difficult because of irregularity of the barrier layer. In the case of anodic films formed on aluminum in chromic acid solution, for example, sever pore blanching appears especially at the higher voltage. Anodic alumina films formed in phosphoric acid and Ematal bath also indicates numerous pore branching and caved pores in the cell wall (2, 3) where anodizing conditions such as high formation voltage and relatively low dissolution ability of electrolytes to anodic oxides are used. These irregularities were caused by defects/holes induced by electric breakdown between pore bottom and protrusion at triple points of cell boundary. Impurity elements in metals and alloys also bring defects and irregularity into porous structures. Furthermore, anodic films formed at some specific condition, especially on titanium, yielded tubular structure having interspaces at cell boundaries. All of these irregularities should be created only at the barrier layer grown under the anodic field by specific reactions other than stable anodic oxide growth. This inhomogeneity of barrier layer such as unevenness of the thickness and voids that induce irregularity in porous oxide seems to be playing an important role at an application for industry both in a positive and negative way. In this paper, we report the specific futures of irregularity appeared in porous anodic oxides films formed on aluminum and evaluated to get unified formation mechanism of such imperfections to be induced in the oxides grown on various substrates. [1] S. Ono, M. Saito, M. Ishiguro and H. Asoh, J.Electrochem. Soc., 151, B473 (2004). [2] S. Ono, H.Ichinose and N. Masuko, J.Electrochem.Soc., 138, 3705 (1991) [3] S. Ono and N. Masuko, J. Jpn Inst. Light Metal, 43, 453 (1993).